The present invention relates to a method and an apparatus for measuring a coefficient of light absorption of substances contained in light scattering materials.
When light is injected in a transparent material, the intensity I of transmitted light is given by the following expression: EQU I=I.sub.0 exp-{.alpha.d} (1)
where I.sub.0 represents the intensity of incident light injected in the transparent material; .alpha. the coefficient of absorption representing the rate of decrease of light due to light absorption per unit thickness of the material; and d the thickness of the material (optical path length). Consequently, such a coefficient of light absorption .alpha. of the transparent material can be obtained, starting from a value obtained by measuring the intensity of light, which has been transmitted through the material, using Eq. (1).
However, for a material having a light-scattering property, light transmitted through the light scattering material is subjected to attenuation due to light scattering, besides attenuation due to light absorption. That is, the intensity I of light, which has been transmitted through the light scattering material, is given by a following expression: EQU I=I.sub.0 exp-{.alpha.d+sd} (2)
where s is a coefficient of light scattering representing the rate of decrease of light due to light scattering per unit thickness of the material.
As can be understood from Eq. (2), since the intensity of light transmitted through the light scattering material varies, depending also on the value of the coefficient of light scattering s, it is not possible to obtain the coefficient of light absorption .alpha., starting only from one measured value of the transmitted light. Therefore, heretofore, apart from the intensity I.sub.1 of the transmitted light for one wavelength, for which the coefficient of light absorption .alpha. is unknown, the intensity I.sub.2 of the transmitted light for another wavelength, for which the coefficient of light absorption .alpha..sub.0 is known, is measured and the unknown coefficient of light absorption .alpha. is calculated by using these measured values. That is, intensities I.sub.1 and I.sub.2 of transmitted light for two different wavelengths are measured and the ratio of these two measured values is formed, as indicated by the following expression. The unknown coefficient of light absorption .alpha. is obtained by eliminating the coefficient of light scattering s in this way. That is, in the present state of the technique, starting from two measured values: EQU I.sub.1 =I.sub.0 exp-{.alpha.d+sd} EQU I.sub.2 =I.sub.0 exp-{.alpha..sub.0 d+sd},
the ratio of the measured values
s EQU I.sub.1 /I.sub.2 =exp-{(.alpha.-.alpha..sub.0)d}
is formed to obtain the unknown coefficient of light 0 absorption .alpha..
In the prior art method described above, it is supposed that the rate of decrease of transmitted light due to light scattering is not varied (equal to each other) for the two wavelengths used for the measurement. However, strictly speaking, this assumption is rarely valid. In particular, in the case where the light scattering power of the material is remarkable, since the value of the coefficient of light scattering s is very great with respect to the value of the coefficient of light absorption .alpha., the difference itself in the coefficient of light scattering s between the two wavelengths is often greater than the coefficient of light absorption .alpha.. For this reason, by the prior art method described above, it was practically difficult to obtain the value of the coefficient of light absorption .alpha. with a high precision.